A BMW dealership I was near burned to the ground when a mechanic doing some fuel tank work screwed up. My hazy understanding is that he removed the tank with ten gallons or so still in it, then dropped it, creating a local fuel-air bomb which was set off when he stepped on a trouble light. I don't believe anyone was seriously hurt but the garage and showroom were totaled.

Got a new exhaust put on my Land Rover yesterday. I'm watching the guys work under mine and other cars, using blow torches and welding equipment, back by where the fuel tanks are.

All I could think of was "just one small leak of fumes and Ka-Fucking-Bm!

I've never been sent to such a call. Does anything like that ever happen?

there's not going to be any "kaboom." For gasoline to cause anything close to a "bang" it needs to be fairly well mixed with air in the proper ratio. and if it's uncontained you'll only get a "whoof."

what can happen when working on an open fuel system is leaked fuel can ignite and cause a large enough fire you can't put it out with a hand-held extinguisher. if it gets to that point, you can only hope the fire department arrives quickly enough to save the building.

Got a new exhaust put on my Land Rover yesterday. I'm watching the guys work under mine and other cars, using blow torches and welding equipment, back by where the fuel tanks are.

All I could think of was "just one small leak of fumes and Ka-Fucking-Bm!

I've never been sent to such a call. Does anything like that ever happen?

Inside the fuel tank, the air/vapor mixture is typically too rich (too much fuel vapor to actually ignite, sort of like leaving the choke on after your lawn mower has already warmed up). When fires happen while refueling a car (generally due to discharge of static electricity), the standard instruction is to just back away; the only thing that's burning is the combustible mixture of air and fuel vapor right at the filler neck (where vapor has been exiting the tank as you fill it with liquid), and the flame won't propagate down the filler neck into the tank. Under these circumstance, things don't generally go badly unless the panicked driver pulls the filler out and sprays liquid fuel all around the general vicinity. The upshot of all this is that performing hotworks on a car's underbody isn't likely to light off anything inside the fuel tank.

Moreover, oxyacetylene torches work pretty fast on the thin metal of an exhaust pipe; any given cut is over pretty fast, so there's not a whole lot of stray warm exhaust gas to dump heat into adjacent components. It would take a dopy, inattentive mechanic to inadvertently point the torch at the surface of the tank for a prolonged period (not saying this has never happened) - and even then I wouldn't expect much of a bang (see previous paragraph).

The only time I would expect a hazard is if a significant quantity of gasoline gets spilled on open ground, where it could conceivably form a combustible mixture and reach an ignition source. Disconnecting a hose from the tank could allow the hose (not the tank) to empty itself on the floor, but this wouldn't be much - only as much fuel as is held inside the hose between the tank and the engine, an ounce or two. If the car is six feet up on a lift, I'm not even sure the flames from such a fire would reach the underbody of the car.

I remember a story some years back about a guy who decided a good way to clean the floor of a garage was to mop the floor with gasoline then use an electric floor buffer to scrub out the oil stains. Kaboom. Fortunately after hours so he was the only one in the building. If true, would have made a good Darwin award but this was before Darwin awards were a thing...

Yeah, the old style split rim truck wheels were well known as "widowmakers."

That's one thing which didn't occur to me... a gasoline leak inside the garage could lead to a build up of vapors, similar to a natural gas leak.

A gasoline leak, assuming you had a decent ambient temperature happening, could be much worse than a nat gas leak as the vapours the gasoline source produced are heavier than air, which would allow them to collect at the floor where your easiest ignition sources are. Typically a floor in a closed room is also not well ventilated, whereas a ceiling is commonly.

You'd need a decent exposed surface area of gasoline to get enough vapor generation to have a kaboom moment though, unless a crazy set of circumstances all align.

Marvin the Martian - didn't see your post but yeh, that'd about do it!

Here's a propane fueled garage that loves the outdoors... It blew the mechanics workshop it was in away.. (It seems that the fact its propane fueled means that was probably the cause .. not the acetylene or other gas used for cutting, welding, heating... or any other fuel or accelerant in the place...)

Yes, it does happen. I remember one case where a man was working underneath an ambulance. There was a gas (not petrol, some other gas) leak and an ensuing explosion. Fortunately everything exploded away from him.

Working on an actual fuel tank is potentially dangerous. Then again a friend of mine worked at a service station when he was a kid, and remembers how one mechanic was very careful when a fuel tank needed repair. He removed the tank, drained it, rinsed it twice with water, and then took to it with a welder. It exploded violently. Didn't kill him, but it wasn't good. Other old hands claimed the safest thing to do was to leave the tank half full of fuel.

Another friend of mine sold her darling little car, and the new owner took it to be checked over, and have some work done on it. They put it up on a hoist, and somehow managed to set the driver's seat alight with a welder. Totalled the car where it sat. My friend was terribly upset.

Then again a friend of mine worked at a service station when he was a kid, and remembers how one mechanic was very careful when a fuel tank needed repair. He removed the tank, drained it, rinsed it twice with water, and then took to it with a welder. It exploded violently. Didn't kill him, but it wasn't good.

Well yes, petrol is insoluble in plain water. You need to use a detergent.

Other old hands claimed the safest thing to do was to leave the tank half full of fuel.

I worked with a guy who claimed his (North Carolina rural white guy) father would routinely weld gas and diesel tanks as long as they were full. The kids always found reason to be somewhere else. LIke South Carolina.

Depending on your definition of explosion, mechanics have been killed due to tires being over-inflated without the use of safety cages. Here's a non-lethal but terrifying example.

I was working on an old Coats 4050A tire machine trying to bust some tires off of some rims and the "demounter" (basically a large, heavy metal prybar sort of thing) suddenly leapt free which, and I am not 100% clear on the physics here, seemed to suddenly transfer all of the force directly to my arm which then bent in a very unnatural manner. The customer, who had been idling nearby observing, immediately remarked "man, you just broke your arm!".

I worked with a guy who claimed his (North Carolina rural white guy) father would routinely weld gas and diesel tanks as long as they were full. The kids always found reason to be somewhere else. LIke South Carolina.

The headspace of a gasoline tank is too rich to explode, and that of a diesel tank is too lean. That's why you NEVER MIX FUEL TYPES!

I torch soldered an emergency repair on a gasoline car tank - empty, but not flushed, and didn't even see a puff from it.

Lifetime mechanic here, the closest I have ever been to getting killed at work was a few months before I retired. I don't remember if it was nitrogen or CO2. They have a pressure relief valve that blows off when they build too much pressure. Very common in hot weather. All the trucks I had worked on in the past had the blow off valve up high. I was under the truck adjust the breaks and it blew off violently, just as I was rolling out from the truck, I would say it missed me by two feet but the gas cloud still almost took me out, I was on the verge of passing out when I got out of it. My face would have turned to an instant ice cycle had I moved one second later.

Working on an actual fuel tank is potentially dangerous. Then again a friend of mine worked at a service station when he was a kid, and remembers how one mechanic was very careful when a fuel tank needed repair. He removed the tank, drained it, rinsed it twice with water, and then took to it with a welder. It exploded violently. Didn't kill him, but it wasn't good. Other old hands claimed the safest thing to do was to leave the tank half full of fuel.

His mistake was to drain it and rinse it; this would have left just enough fuel in the tank to provide enough vapour to form a nice explosive mixture.

My grandfather ran a mechanical engineering workshop in a rural area and he refused to do hotwork on fuel tanks which cost him business. Most workshops would fill tank with water, to overflowing, before welding on them. This is very safe unless - as has happened more than once - there is a little bit of fuel vapour trapped in a folded seam of the tank. It is of course the seams which tend to require re-welding.

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